Optical Time-Domain Reflectometry (OTDR—also used to refer to the corresponding device) is a diagnostic technique for optical fiber networks where a light pulse is launched along an optical fiber link and the returning light is detected and analyzed. Various events along the fiber link can be detected and characterized through a proper analysis of the returning light in the time domain.
Performing a measurement with traditional OTDR requires the user to specify settings such as pulse characteristics, acquisition range (i.e. the distance light travels within the fiber) and averaging time. A single acquisition is usually performed under the selected user settings. Alternatively, more than one acquisition may be performed by the OTDR within the specified acquisition time, all under the same user settings and therefore using the same pulsewidth, but with different gain settings, receiver bandwidth or pulse power for example. Acquired data from each sub-acquisition is then stitched together, according to their respective noise floor and saturation levels, to build a single graphical x-y representation of the backscattered light referred to as the “trace”.
Most OTDRs also provide an automatic mode, where the instrument automatically chooses one appropriate pulse, acquisition range and averaging time. In order to choose the appropriate settings for the final acquisition, the instrument launches one or many short “investigation acquisitions”, which provide a quick overview of the link being tested. In general, the investigation acquisitions are hidden from the user, and only the final acquisition is made available.
For both manual settings and automatic settings, the final result is an OTDR trace performed with a single pulse. In general, the pulsewidth will be selected to allow characterization of the complete link. For example, a link with large loss will end up being tested with a long pulse. However, the use of a long pulse brings certain limitations in the ability to characterize short fiber sections, as well as closely spaced events.
An improvement to the single-pulse approach has been developed, whereby the equipment makes use of successive acquisitions performed with increasingly larger pulses. Such an approach is the basis of the Intellitrace PIus™ technology by Tektronix (see U.S. Pat. Nos. 5,155,439 (HOLMBO et al) and 5,528,356 (HARCOURT)). Shorter pulses are used to characterize the near end of the fiber under test. A second acquisition with a larger pulse is then taken, to characterize the portion of the link-under-test that is farther away. The process of taking a new acquisition with a longer pulse is repeated until the end of the fiber under test is found. The information obtained from the different acquisitions is combined to produce a single result; that is, a single composite OTDR trace and/or single event table in which each event is measured using the acquisition that was performed with the smallest pulse possible (i.e. the acquisition that provides a sufficient SNR to perform loss/location/reflectance measurements within a target accuracy). It is to be noted that the number of pulses that is used depends on the link-under-test (only one pulse for a short link, many pulses for a long link). The successive acquisitions can be performed in a dynamic manner, or using a “fixed recipe”, that is, always testing with a given sequence of pulses. In practice, differences in gain settings, filtering, bandwidth, etc. may occur for each acquisition.
The “sequential pulses” approach brings a significant improvement to the traditional single-pulse approach, as each event can be characterized by an “optimum” pulse. However, certain drawbacks remain: for example, the optimum pulse for measuring loss is not necessarily the same as the optimum pulse to measure reflectance or to perform event location. Moreover, situations exist where a single pulse cannot characterize an event. There therefore remains a need for an improved OTDR method and device.